This invention relates to production of germanium-containing glass. The invention is more particularly concerned with an improved glass manufacturing facility and method of operation, wherein germanium-containing particulate filtered from an exhaust gas stream of the glass production process is collected from a filtration system at a concentration level conducive to reclamation of the germanium.
In the manufacture of germanium-containing glass for various advanced optical products, such as optical fibers, germanium dioxide (GeO2) particulate, silicon dioxide (SiO2) particulate, hydrochloric acid (HCl) fumes, and water vapor are produced as byproducts. These byproducts are exhausted to a pollution abatement system where, among other things, the GeO2 and SiO2 particulate are captured from the exhaust stream and collected for disposal in a landfill.
FIG. 1 is a basic block diagram of a conventional glass manufacturing facility 10 that produces germanium-containing glass. The facility comprises a glass
production system 12 including a plurality of lathes 14. The lathes produce bodies of GeO2-doped silica glass, commonly called blanks or preforms, using a process known as flame hydrolysis. The aforementioned byproducts are reaction products of this process.
In the lathes 14, vaporous SiCl4 and GeCl4 as raw materials are passed though a specially designed methane burner in precisely controlled amounts depending upon the desired constitution of the silica glass blanks. The SiCl4 and GeCl4 are reacted with oxygen under the heat of the burner to form minute particles or xe2x80x9csootxe2x80x9d of SiO2 and GeO2. A portion of these particles is deposited on the outer periphery of a rotating mandrel (a technique known as outside vapor deposition or OVD) to form the glass blanks. The excess soot is exhausted from the lathes.
The respective exhausts of the lathes 14 are connected to a dedicated loop L that pulls air from the lathes by way of a loop exhaust fan 16. The fan circulates the exhaust stream to a baghouse 18, including a plurality of baghouse modules 20 (four in the form shown), for filtration of the SiO2 and GeO2 particulates from the stream. After filtration by the baghouse 18, a portion of the exhaust stream is recirculated through the loop and mixed with pre-heated makeup air. The non-recirculated portion of the exhaust stream is supplied, via a scrubber fan 22, to a scrubber system 24 and then discharged through an exhaust stack 26. The scrubber system 24 scrubs the filtered exhaust stream from the baghouse with weak acid or soft water to remove vapor-phase chloride components, including HCl, SiCl4 and GeCl4, to ensure that the discharge from the exhaust stack complies with environmental requirements.
In conventional operation of the production system 12, different types of blanks are run on individual lathes depending upon customer demand. At any given time, some of the lathes 14 may be running a low concentration of germanium, while others may be running a high concentration of germanium, with still others running at intermediate concentration. GeO2 content may differ between individual blanks by as much as about 13-14% by weight, and possibly more. It will thus be appreciated that the GeO2 concentration in the exhaust stream supplied to the baghouse system varies widely, depending upon the particular mix of blanks being manufactured a given time.
It should be noted, incidentally, that the earlier-described production process of the lathes 14 is merely exemplary. As is well known to those skilled in the art, the flame hydrolysis process can be implemented using materials other than those mentioned above. And soot deposition can be accomplished by other techniques such as modified chemical vapor deposition (MCVD), in which soot is deposited on the inner periphery of a rotating hollow mandrel, and vapor axial deposition (VAD), in which soot is deposited on the axial end of a rotating rod. Indeed, as shown in FIG. 1, the glass production system 12 includes additional lathes 28 that produce blanks of germanium-free silica glass, and these lathes are also connected to the loop L.
The SiO2 particulate and the GeO2 particulate byproducts from the lathes 14 are generally in the size ranges of 0.5-1 xcexcm and 0.05-0.5 xcexcm, respectively. To capture these particulates, as well as the particulates from the lathes 28, the baghouse modules 20 use acrylic filter bags 21 that are pre-coated (pre-loaded) with particulate SiO2. The exhaust stream from the lathes is passed through the pre-coated bags which trap and accumulate substantially all of the particulate material entrained in the exhaust stream. Each of the baghouse modules is periodically taken off-line and isolated from the loop L in order to conduct a cleaning cycle. During the cleaning cycle, a mechanical shaking system (not shown) shakes the bags of the isolated module to dislodge the accumulated particulate material captured from the exhaust stream. The cleaned bags are then recoated with SiO2 from a supply system 30 connected to the baghouse, and thereafter placed back on-line.
The material dislodged from the filter bags during the cleaning cycle is collected and landfilled in an appropriate facility. By landfilling the particulate mixture collected from the baghouse, there is substantial waste of costly germanium.
Technology exists and is commercially available for reclaiming germanium from particulate GeO2. But, in the conventional facility using pre-coated bags and operated as described above, the concentration level of germanium in the material collected from the baghouse is insufficient for cost-effective reclamation. As a practical matter, a minimum concentration of 2% germanium by weight is ordinarily required.
In accordance with the present invention, it has been discovered that by appropriate adaptation of the manufacturing facility design and operation, it is possible to collect a substantial portion of the heretofore wasted germanium in sufficient concentration to permit cost-effective reclamation. In particular, it has been discovered that by modifying the baghouse to use filtration bags having a PTFE (polytetrafluoroethylene) filter membrane supported on a PTFE backing fabric, and by dedicating the glass production system connected to the baghouse to the production of blanks having sufficient germanium concentration, it is possible to obtain a particulate mixture from the baghouse that meets or exceeds the aforementioned 2% level. For example, in a preferred mode of the invention to described in detail later, the production system is constituted by lathes which are dedicated predominantly to the production of blanks for use in making multi-mode optical fibers.
Briefly stated, in accordance with one of its broader aspects, the invention provides a glass manufacturing facility which comprises a glass production system providing an exhaust stream entrained with particulate material including germanium-containing particulate, and an exhaust filtration system including PTFE membrane filter material supported by PTFE fabric material, the exhaust filtration system being connected to the glass production system to receive the exhaust stream and capture the particulate material. The facility additionally comprises a collection system connected to the exhaust filtration system to collect the captured particulate material from the exhaust filtration system. The glass production system operates to produce glass selected such that the concentration of germanium in the particulate material collected by the collection system is at least about 2% by weight.
In a related aspect, the invention provides an operation method of a glass manufacturing facility, which comprises producing glass with a glass production system that provides an exhaust stream entrained with particulate material including germanium-containing particulate, filtering the exhaust stream with an exhaust filtering system including PTFE membrane filter material supported by PTFE fabric material, thereby capturing the particulate material, and collecting the captured particulate material from the exhaust filtration system with a collection system. As previously stated, the glass is selected such that the concentration of germanium in the particulate material collected by the collection system is at least about 2 1% by weight.